Tonight's verdict
Why one number beats four
Most aurora apps show you the KP index (planetary magnetic activity, 0–9) and stop there. KP 4 in Reykjavík with 80% cloud cover is a wasted night. KP 3 with 5% clouds and a new moon is a magical one. Same prediction engine, different real outcome.
This estimator multiplies the four relevant factors — magnetic activity, sky clarity, lunar interference, and your latitude's geomagnetic line — into a single 0–100% probability. It can't tell you the aurora will definitely appear; it tells you whether going out is worth it.
FAQ
What KP is "good enough" in Iceland?
KP 3+ is enough in northern Iceland; KP 4+ works in Reykjavík. Iceland's latitude means even moderate KP produces visible auroras on a clear night.
Where should I go from Reykjavík?
Away from the city's light dome: Grótta lighthouse (15 min west), or any of the south coast pull-offs. Going to Þingvellir (40 min) doubles your chances by adding horizon and removing light pollution.
Is the aurora visible in summer?
No — there is no astronomical darkness between mid-May and mid-August. Aurora season is late September to late March.